Process for preparing magnetic disk

ABSTRACT

This invention relates to a process for preparing a magnetic disk, which comprises (a) subjecting a substrate having an anodized aluminum film to mirror surface-finishing, (b) widening pores of the anodized aluminum film by chemical dissolution treatment so that the total area of pores becomes from 20 to 80% of the entire surface area, thereby retaining crystalline alumina of the anodized aluminum film extruded after the chemical dissolution treatment, and (c) coating the resultant substrate with a magnetic continuous thin film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation in part of U.S. Pat. application Ser.No. 07/250,088 filed Sept. 2, 1988, now U.S. Pat. No. 4,925,738, andincorporated entirely herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing a magnetic diskby forming a magnetic continuous thin film on the surface of asubstrate. More particularly, the present invention relates to a processfor preparing a magnetic disk improved by dissolving problems concerningthe friction coefficient of the surface and the sticking to a magnetichead.

A magnetic recording medium using a magnetic continuous thin film(hereinafter referred to as a "magnetic thin film") as a high densityrecording material is not widely used because of the problems in respectof mechanical durability and the like.

Namely, a recording medium using a magnetic thin film and having a verysmooth surface is likely to lead to sticking when brought in contactwith a magnetic head. Further, a liquid lubricant applied to the mediumis easily removed by the contact with the magnetic head, whereby thefriction coefficient increases, thus leading to head crush.

In order to avoid such problems, a mechanical texture method has beenattempted in which scratch marks are mechanically imparted to thesurface of the substrate by means of e.g. sand paper. However, it isvery difficult to impart such scratch marks while controlling not toincrease bit errors and not to cause the sticking to the magnetic head.Further, in the mechanically scratched texture, fine burrs exist on thesurface, and they tend to peel off upon collision with the magnetic headand thus lead to head crush.

Japanese Unexamined Patent Publication No. 22220/1984 discloses theinvention relating to a process for preparing a substrate for a magneticdisk, the improvements of which are to reduce the friction coefficientof the surface of the magnetic disk having a magnetic thin film formedon the substrate and to reduce a sticking force to a magnetic head.

This process comprises subjecting the surface of an aluminum alloysubstrate coated with an Alumite layer to mirror surface-finishing, andetching the mirror surface-finished surface to form concave parts havinga surface precision of Ra 70-1,400 Å.

The mirror surface-finishing in the cited process can be conducted byabrading, rubbing, polishing or other methods, thereby forming themirror surface-finished surface having R_(max) of at most 200 Å and Raof at most 50 Å, but in this state, the sticking of a magnetic headoccurs. In order to avoid the sticking of the magnetic head, the surfaceis etched to form convex parts in such manner as to have a surfaceprecision of Ra 70-1,400 Å, but this process still has the followingproblems.

Firstly, the pore area ratio (ratio of the total area of pores to theentire surface area) is constantly about 10% before forming a magneticthin film since the pores of the Alumite layer are not widened by themirror surface-finishing and etching steps. Consequently, the contactingarea of a magnetic head is relatively large, and it is thereforenecessary to enlarge the surface roughness in such manner as to reducethe friction coefficient.

Secondly, the etching is plasma-etching carried out in the atmosphere of0₂, Ar₂ or a mixture thereof, and the plasma-etching abrades particlesout of the Alumite layer surface by bumping plasma against the Alumitelayer. Accordingly, after etching, the surface becomes rough and thesurface roughness remarkably varies due to the presence of the abradedpowder (SiO₂, Al₂ O₃) in Alumite pores and the impurities (Fe₃ 3Si andthe like) in the Alumite film, which respectively have different etchingrates.

In other words, according to the above process, the plasma-etching isemployed to secure the required surface precision taking the small porearea ratio into consideration, but the surface roughness after etchingremarkably varies, which consequently leads to large variation in thefriction coefficient over the entire surface of the magnetic disk. Thus,it is difficult to prevent the sticking of a magnetic head and the headcrush, and it is also difficult to reduce the spacing between themagnetic head and the magnetic disk.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above mentionedproblems by providing a magnetic disk having a magnetic thin film on thesurface of a substrate, said substrate having an anodized aluminum filmtreated by chemical dissolution different from the knowntexture-treating method after mirror surface-finishing in such manner asto form a texture structure completely different from those of the knownsubstrates, thereby forming uniform surface roughness so as to reducethe friction coefficient of the magnetic disk to a magnetic head and toprovide constantly stable friction coefficient over the entire surfaceof the magnetic disk.

Thus, the above object can be achieved by providing a process forpreparing a magnetic disk, which comprises (a) subjecting a substratehaving an anodized aluminum film to mirror surface-finishing, (b)widening pores of the anodized aluminum film by chemical dissolutiontreatment such as electrolytic etching so that the total area of poresbecomes from 20 to 80% of the entire surface area, thereby retainingcrystalline alumina of the anodized aluminum film extruded after thechemical dissolution treatment, and (c) coating the resultant substratewith a magnetic continuous thin film.

It is preferable to widen the pores of the anodized aluminum film to adepth of 50-10,000 Å, preferably 50-500 Å.

It is also preferable that the anodized aluminum film has a (111)crystal face as the predominant face.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a magnified plan view showing a part of an anodized aluminumfilm

FIG. 2 is a section view along the line II--II of FIG. 1.

FIG. 3 is a graph showing the measurement result by a surface roughnessmeasurer with regard to Sample No. 1 shown in Table 1.

FIG. 4 is also a graph showing the measurement result of the surfaceroughness of Sample No. 4 by the same surface roughness measurer.

FIG. 5 is a graph showing the measurement result by STM with regard toSample No.3.

FIG. 6 is a three dimensional picture by STM with regard to the magneticdisk prepared by using Sample No. 3.

FIG. 7 is a graph showing the relation between the effective area ofpores and the friction coefficient of the substrate surface.

FIG. 8 is a graph showing the relation between the depth of pores andthe friction coefficient.

FIG. 9 is a microscopic photograph of the metal texture showing thesurface state of a magnetic disk prepared in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The pores of the anodized aluminum (Alumite) film on the substrate ofthe magnetic disk of the present invention are widened by chemicaldissolution. The crystal structure of the anodized aluminum film has athree-layered structure comprising crystalline alumina located outermostfrom the inner wall of a pore, amorphous alumina and amorphous aluminacontaining acid or alkali ions located innermost of the pore. Thecrystalline alumina is hardest to be dissolved, thus remaining afterchemical dissolution treatment so as to provide quite uniformdistribution of extruded parts on the surface of the substrate. Amagnetic thin film subsequently formed on the surface of the substrategrows faithfully along the above extruded parts.

Thus, the pore-widening treatment by chemical dissolution achieves notonly the widening of the pore diameters but also the formation ofuniformly distributed extrusion parts of the crystalline alumina, whichcontribute to the reduction of the area of the surface in contact with amagnetic head. In addition to the above advantages, the widening ofpores achieves an effect to remove the abrasion residues in the pores.

The surface roughness of the substrate formed by texture-treatment bychemical dissolution in accordance with the present invention is uniformover the entire surface, and does not substantially vary depending onthe length of treating time.

In this manner, the pore area ratio is adjusted in such manner as thatthe total area of pores constitutes from 20 to 80% of the entire surfacearea, and the abrasion residues in pores are removed. Consequently, thearea of the magnetic disk surface constituted by the extruded parts ofthe crystalline alumina in contact with a magnetic head becomes verysmall, and an extremely thin air film is formed between the magneticdisk and the magnetic head, whereby the friction coefficient between themagnetic head and the magnetic disk is remarkably reduced.

Especially when the depth of pores is selected suitably within a rangeof from 50 to 10,000 Å, the friction reducing effect is ensured, and theretention of a lubricant is improved so that the friction coefficientcan be maintained at a low level for a long period of time.

The process for preparing a magnetic disk in accordance with the presentinvention is effective for reducing costs in mass production since it isonly required to conduct pore widening treatment in an electrolytic bathfor dissolution after usual anodic oxidation treatment of aluminum byusing the above-mentioned material and process, without requiring anyspecial packing agent.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted by such specific Examples.

EXAMPLES

In a first step, an aluminum coating layer having a thickness of 1 μmwas formed on the surface of a glass substrate by vacuum vapordeposition.

In a second step, the above substrate was subjected to anodic oxidationtreatment in a 3% oxalic acid aqueous solution under an applied voltageof 50 V to form an anodized aluminum (Alumite) coating layer having apore diameter of 370 Å, a cell size of 1,100 Å and an effective area ofpores of 9%.

In a third step, after subjecting the above substrate to ordinary mirrorsurface-finishing, the resultant substrate was subjected to porewidening treatment by dipping the substrate in a 10% H₃ PO₄ electrolyteat 30° C. for chemical dissolution.

By adjusting the treating time for the third step, 6 samples havingvarious effective areas of pores as shown in Table 1 were obtained.

                  TABLE 1                                                         ______________________________________                                        Sample No.                                                                             1       2       3     4     5     6                                  ______________________________________                                        Treating  0      20      40    60    80     100                               time (min.)                                                                   Pore     340     480     600   750   870   1000                               diameter                                                                      (Å)                                                                       Effective                                                                               9      17      27    42    57     75                                area (%)                                                                      ______________________________________                                    

As can be seen from FIGS. 1 and 2, an anodized aluminum film generallyhas a three-layered crystal structure comprising crystalline alumina(c), amorphous alumina (b) and amorphous alumina (a) containing an acidor alkali ion, the crystalline alumina (c) being located outermost froma pore (P) and the amorphous alumina (a) being located innermost in thepore (P). Among the three layers, the crystalline alumina (c) is hardestto be dissolved, thus remaining extruded when pores are widened by thechemical dissolution. Consequently, a contacting area of a magnetic headwith a magnetic disk using this substrate is remarkably small due to thepresence of the widened pores and the extruded parts of the crystallinealumina (c) as compared with conventional products.

As can be seen from FIGS. 3 and 4, surface roughness achieved bysubjecting pores of an anodized aluminum film to the texture treatmentby chemical dissolution does not vary depending on the length of thedissolution treating time. FIGS. 3 and 4 are graphs showing themeasurement results of surface roughness of Sample Nos. 1 and 4 shown inTable 1 by a contacting type surface roughness measurer (manufactured byTensor Instruments, U.S.A.).

FIG. 5 is a graph showing the measurement results of surface roughnessof Sample No. 4 by a scanning tunnel microscope (STM).

The concavo-convex pitches of the surface roughness of the substrate ofthe present invention can not be numerically measured by an ordinarysurface roughness measurer since the concavo-convex pitches are in cellsize unit (1,000 Å). However, as can be seen from FIG. 5, according tothe scanning tunnel microscope, the concavo-convex (roughness) of thesurface was 200-150 Å, thus being very uniform. This is due to the factthat the pore distribution of an anodized aluminum (Alumite) film isvery uniform and the dissolution rate of the anodized aluminum film bychemical dissolution does not remarkably vary. The dissolution rate ofthe same layer of the three layered structure is equal although thedissolution rate varies a little depending on each layer.

FIG. 6 shows a three-dimensional picture by a scanning tunnel microscope(STM) with regard to a magnetic disk prepared by coating Cr (2,000 Å),Co-Ni-Cr (550 Å) and C (300 Å) on the Sample No. 3 substrate shown inTable 1 by sputtering. It can be understood that the concavo-convexpattern of R_(max) 200 Å is formed on the surface.

Thus, the surface roughness of the disk prepared in accordance with thepresent invention is much more uniform as compared with the surfaceroughness of R_(max) 1,100 Å achieved by plasma etching. Consequently,the space between a magnetic head and the magnetic disk of the presentinvention can be remarkably reduced, thus enabling high densityrecording.

FIG. 7 shows the change in the friction coefficient when the porediameter was changed to change the effective area of pores whilemaintaining the cell size to be constant, as mentioned above.

For the measurement of the friction coefficient, a substrate having acarbon coating layer of 200 Å applied after the pore widening treatmentto harden the substrate surface was used. The friction coefficient inthe case where a liquid lubricant is applied to the substrate surfacehaving enlarged pores showed a constant value of 0.2 in thepredetermined range of the effective area of pores, and the lineconnecting the measured values became a horizontal straight line.

As is evident from FIG. 7, the friction coefficient increases if theeffective area of pores is less than 20%. This is because the stickingforce of the medium to the magnetic head increases. Further, if theeffective area of pores exceeds 80%, the substrate surface tends to bebrittle, whereby the friction coefficient likewise increases.

By adjusting the electric amount for the formation of anodized aluminum(Alumite) film, the pore depth was varied, and the relation between thevarious pore depths and the friction coefficients was investigated. Forthe measurement, a substrate having a carbon coating layer of 200 Å onits surface and a magnetic head made of Mn-Zn alloy having a weight of15 g were used. The results of the measurement are shown in FIG. 8.

As is evident from the Figure, the friction reducing effect appears at alevel of about 50 Å. If the depth of pores exceeds 10,000 Å, thefriction coefficient tends to increase again since a lubricant, etc. arelikely to enter the pores. Further, when the depth of pores exceeds 500Å, bit errors tend to increase. For this reason, the depth of pores ispreferably from 50 to 500 Å.

In the above Examples, the coating layer was formed by vapor depositionof aluminum on the surface of a glass plate, and then subjecting thelayer to anodic oxidation treatment (Alumite treatment), and thisrepresents merely one example of the hard substrate and the method forforming an aluminum coating layer. As the hard substrate, besides aglass plate, an aluminum alloy plate may be employed. Further, for theformation of an aluminum coating layer, sputtering or any other knownmethods may be employed in place of vapor deposition method. An aluminumor aluminum alloy substrate may be directly subjected to anodization(Alumite) treatment without forming an aluminum film thereon.

As mentioned above, one of the objects of the present invention is toovercome the problem of magnetic head crush by utilizing thephysicochemical properties of the anodized aluminum coating layer.Namely, by virtue of the smoothness of the coating layer surface, headcrush is prevented, while the pores are widened by dissolution to adjustthe effective area of the pores to a proper level and a small contactingarea is secured by uniform concavo-convex surface formed by preparingextruded parts of crystalline alumina of three-layered structure onAlumite film, so that the friction coefficient can be reduced.

From this viewpoint, for the aluminum coating layer of the substrate ofthe present invention, it is ideal to use an aluminum coating layerhaving a (111) crystal face as the predominant face. The (111) face ismost excellent in the surface precision. Accordingly, when an aluminumcoating layer having such a (111) face as the predominant face is usedfor the substrate, the smoothness of the surface will be excellent,whereby the problem of the head crush will be completely solved.

A magnetic disk of the present invention is prepared by depositing amagnetic material of Co-Cr alloy or Co-Ni alloy on the substrate surfaceafter the pore widening treatment, by sputtering, plating or other knownmethod. Such a magnetic thin film gows following the surface roughnessof the substrate, whereby the texture structure provided by the enlargedpores is maintained.

FIG. 9 is a microscopic photograph of a magnified ratio showing thestate of the surface of a substrate coated with a magnetic thin film andfurther with a carbon layer.

When an aluminum alloy substrate is anodized (Alumite-treated) and thepores of the Alumite film is widened, it is preferably that the Alumitefilm has a thickness of not less than 4 μm. In this case, when the depthof the pores is large, the bottoms of the pores may optionally be filledup with a lubricant to retain the lubricant on the surface of thesubstrate. However, it is related with the viscosity of the lubricantused as to whether the bottoms of the pores should be filled up or not.

As described in the foregoing, according to the present invention, thetexture is obtained by controlling the proportion of the area of poreson the surface of the anodized aluminum (Alumite) coating layer bychemical dissolution, and therefore, the concavo-convex surface isremarkably uniform as compared with the texture prepared by theconventional plasma etching method. Consequently, the frictioncoefficient is stabilized on the entire surface and head crush scarcelytakes place.

I claim:
 1. A process for preparing a magnetic disk which comprisesforming a magnetic continuous thin film on a concavo-convex roughenedsurface of an aluminum film, said method of forming a magnetic thin filmconsisting essentially of (a) subjecting a substrate having an anodizedaluminum film having a three-layered structure comprising crystallinealumina, amorphous alumina and amorphous alumina containing an acid oralkali ion to mirror surface-finishing, (b) treating the anodizedaluminum to chemical dissolution treatment thereby widening pores of theanodized aluminum film so that total area of pores becomes from 20 to80% of the entire surface area and that the depth of said pores becomesfrom 50 to 500 Å, thereby retaining crystalline alumina of the anodizedaluminum film extruding after the chemical dissolution treatment,whereby a concavo-convex surface is formed on said anodized alumina filmand (c) coating said concavo-convex surface resulting from said chemicaldissolution treatment with a magnetic continuous thin film directly onsaid anodized aluminum film subjected to chemical dissolution treatment.2. The process according to claim 1, wherein said chemical dissolutiontreatment is electrolytic etching.
 3. The process according to claim 1,wherein the substrate is further coated with a carbon layer.
 4. Theprocess according to claim 1, wherein the substrate is a glass plate, analuminum plate or an aluminum alloy plate, having an anodized aluminumfilm.
 5. The process according to claim 1, wherein the magneticcontinuous thin film comprises a Co-Cr alloy or a Co-Ni alloy.
 6. Theprocess according to claim 1, wherein the anodized aluminum film has athickness of at least 4 μm.
 7. A process for preparing a magnetic disk,which comprises (a) subjecting a substrate having an anodized aluminumfilm having a (111) crystal face as the predominant face to mirrorsurface-finishing, (b) widening pores of the anodized aluminum film bychemical dissolution treatment so that the total area of pores becomesfrom 20 to 80% of the entire surface area, thereby retaining crystallinealumina of the anodized aluminum film extruded after the chemicaldissolution treatment, and (c) coating the resultant substrate with amagnetic continuous thin film.